The pictures of Prince Harry cavorting naked in a Las Vegas hotel room were irresistible to any self-respecting tabloid editor. By printing them the Sun will have entertained its readers and added to its sales. Both are legitimate reasons for publication, but they are not the whole story.

The public interest argument in favour of publication goes like this: Prince Harry is third in line to be head of state. Under a monarchical system we have no choice in who rules over us, but, as citizens, we have a reasonable curiosity about the character of those who could plausibly be king or queen. The prince's security arrangements are paid for by the taxpayer. Finally, it is questionable whether there is genuine privacy in a situation in which a prominent public figure invites total strangers to his hotel room for a game of strip billiards.

The Sun used some of these arguments in defence of publication, but pinned its main focus on the right to print material that was already circulating in the public domain. It drew comparisons with the affair between Edward VIII and Wallis Simpson – a story of genuine public import, discussed openly in America, but not at all in the British press.

There are strong counter-arguments. Prince Harry is a single man and he has a right to a private life. What he does in the privacy of his own hotel room is his own business, and no one else's, so long as it is legal and consensual. The argument that the material is already in the public domain is a strong one – but also dangerous. Taken to its logical extremes, it means Britain abandons any kind of ethical, regulatory or legal framework. The stage is thus set for a balancing of two human rights – freedom of expression versus privacy. On the facts of this particular case it is a fine balance.

Who should decide? Until a generation ago the answer would have been simple: the editor and the publisher – and they alone. But that – as Elisabeth Murdoch conceded in the MacTaggart lecture this week – is no longer a tenable argument. Rupert Murdoch's tabloids have, by their actions, undermined any public trust in allowing editors the complete freedom to do as they deem fit. The Sun surely understands that.

So, in such cases, the courts and the press regulator will almost certainly have a role. If newspapers want the courts to keep their noses out of these issues they will have to work with Lord Justice Leveson to create a regulator which can reach fair and swift decisions in cases where there is a tension between freedom of expression and privacy. In the end, the only reasonable test is not entertainment or sales – but an agreed concept of the public interest.